Surface acoustic wave filter arrangements of this kind are already known. The FIG. 1 shows a portion of such a network realized in the shape of a ladder network, the circuit diagram of which is shown on FIG. 2. On this figure, between an input terminal 1 and an output terminal 2 a plurality of seriesly connected impedance elements 3 are shown with a parallel impedance element 4 between respectively the input terminal 1, the output terminal 2 and the nodes 5 between two adjacent series impedance elements 3 on the one hand, and the ground 6 on the other hand. On the FIG. 1, the series impedance elements 3 are realized as upper and lower metallic strips forming bus-bars 7, 8 deposited on a piezoelectric substrate (not shown). The parallel impedance elements 4 are realized as inter-digital electrodes 9, 10 extending perpendicularly respectively from the upper bus-bar 7 and the lower bus-bar 8.
The FIGS. 3 and 4 illustrate the electrical behaviour of a ladder network in accordance to the FIGS. 1 and 2. FIG. 3 shows the simulated impedance performance Zi/Zo of impedance elements 3 and 4, Zi and Zo being the input and output impedances. The first impedance element 3 has a low impedance of for instance 1 Ohm, at a resonance frequency f1 and a high impedance of for instance 1000 Ohms, at an anti-resonance frequency f2. The second impedance element is shifted in frequency so that the resonance frequency f1 of the series impedance element 3 and the anti-resonance frequency f4 of the parallel impedance element 4 were about the same.
The simulation of this impedance element arrangement yields the electrical bandpass filter performance such as shown on FIG. 4 for a Γ-type scheme with a passband around f1 and f4 and deep notches at f2 and f3. This figure shows the dependency of the amplitude AM in dB from the frequency f in MHz.
In practice the real impedance performance differs from the illustrated simulated performance, particularly due to parasitic effects. One of these is caused by the electrostatic interaction between the bus-bars 7 and 8 and the edges 12 of the inter-digital electrodes 9 and 10. This electrostatic interaction creates a periodic charge and electric field distribution in the bus-bar area 13 adjacent the corresponding inter-digital electrodes edge 12 and in the gap 14 with the same periodicity as for inter-digital electrodes 9, 10. On the FIG. 4 the electric field distribution is illustrated by arrows. Both the gap electrical field and the bus-bar charge distribution create a parasitic acoustic resonance on a higher frequency, the frequency shift and the amplitude of which depend on substrate material, metal thickness, mark-to-period ratio and is usually in a range of 0.01 to 1% for the frequency shift and in a range of 0.1 to 10% for the amplitude. Due to this parasitic interaction the real passband of an impedance element filter is narrower than the simulation made without taking into account the parasitic interaction.
The FIG. 5 illustrates the frequency response FR of a 947.5 MHz surface acoustic wave filter whereon the dashed line indicates measured values and the solid line the simulated filter response. It is to be noted that the 3 dB level L of the measured curve has a width of 5 MHz less than the simulated curve, on the lower frequency side of the centre frequency. In the filter which has been used the parallel impedance elements 4 were about four times longer than the length of the elements 3. When the length of the series impedance elements 3 is greater than the length of the parallel elements 4, the main difference between the measured and the simulated curve is on the side of the higher frequencies with respect to the centre frequency of the passband.
A second parasitic effect resides in the wave-guide mode excitation.